1. Field of Invention
The present invention relates to an apparatus for measuring a wavelength dispersion characteristic of DUT (Device Under Test) such as a combination of an optical fiber and an optical amplifier.
2. Description of the Related Art
It may cause a great loss of light when light is transferred to a long distance only through an optical fiber. Therefore, the loss is prevented by using an optical fiber line combined with an optical amplifier (EDFA) which amplifies an optical signal therein. The optical amplifier passes light to a unilateral direction. The optical fiber line means a combination of an optical fiber and an optical amplifier.
FIG. 7 shows a construction of a measuring system for measuring a wavelength dispersion characteristic of an optical fiber line. An optical fiber line 110 is formed by a combination of an optical fiber 112 and an optical amplifier 114. The optical fiber line 110 passes light into a right direction. A variable wavelength light source 12 generates light by changing a wavelength. The light is modulated to a frequency of a power supply for modulation 14 by an optical modulator 15, and then is incident upon the optical fiber line 110. The light transmitting the optical fiber line 110 is converted into an electric signal by photoelectric converter 22, and a phase thereof is compared with that of an electric signal generated from the power supply for modulation 14 by a phase comparator 24. That is, a phase difference is calculated. It is possible to obtain a group delay or a wavelength dispersion of the optical fiber line 110 from the phase difference.
In addition, generally, light with the condition of WDM (wavelength division multiplying) is inputted into the optical fiber line 110. On the premise of this, the optical fiber line 110 is designed to maintain a quality of transferred waveform of the optical fiber line 110 by incidence of light, such as 16 waves and 40 waves and the like, which has a wavelength magnification in accordance with a design upon the optical fiber line 110. Therefore, each optical amplifier 114 sets an automatic gain feedback to maintain a fixed level of an outgoing light.
At this point, as shown in FIG. 8, there is a case that an optical wavelength in a certain specific narrow range is incident upon the optical fiber line 110. FIG. 8(a) shows a relationship between wavelength xcex and power P, and FIG. 8(b) shows a relationship between time t and the power P. As shown in FIG. 8(a), the power is increased at a certain wavelength xcex0. In this case, as shown in FIG. 8(b), there is no distortion in a waveform.
FIG. 9 shows an outgoing light in case that the light of an optical wavelength in a certain specific narrow range is incident upon the optical fiber line 110. FIG. 9(a) shows a relationship between wavelength xcex and power P, and FIG. 9(b) shows a relationship between time t and power P. As shown in FIG. 9(b), a distortion occurs in an output waveform. That means a time jitter of signal. This is derived from the following reasons.
That is, when the optical wavelength in certain specified narrow range is incident upon the optical fiber line 110, an amplifier gain of the optical fiber line 110 is changed over a range estimated at the time of designing. Accordingly, the light of a certain wavelength xcex0 is propagated with a strong power that is over the level estimated at the time of designing. When the powerful light is propagated the optical fiber line 110, the optical fiber line 110 is put into a non-linear region. Due to the non-linearity of the optical fiber line 110, an index of refraction of the optical fiber line 110 is varied with the lapse of time according to the optical power, the distortion of the output waveform is caused.
As the output waveform is distorted, an error is caused when detecting a phase difference from the phase comparator 24. Accordingly, the group delay or the wavelength dispersion of the optical fiber line 110 which are obtained from the phase difference may generate the error.
Moreover, it may be possible to minimize the distortion of the output waveform by adjusting a gain characteristic of the optical fiber line 110, however, it is difficult to adjust the gain characteristic.
Therefore, it is an object of the present invention to provide an apparatus for reducing a waveform distortion of an outgoing light in case that light of an optical wavelength in a certain specific narrow range is incident upon DUT.
In an optical characteristic measuring apparatus so constructed when incident light of a wavelength in a narrow range is supplied to the light transmitting objective, a distortion in the waveform of a transmitted light is caused. However, by adjusting the output of the incident light, a SIN ratio (signal to noise ratio) is lowered. The noise is of a relatively wide range of wavelength. Accordingly, if the S/N ratio is lowered properly, the incident light of a wavelength in the relatively wide range can be supplied to the objective. Therefore, it is possible to reduce a waveform distortion of the outgoing light.
In addition, there is an optical fiber, or a combination of an optical fiber and an optical amplifier for use as the light transmitting objective.
In accordance with the present invention, the incident light supplying unit includes a variable wavelength light source for generating light of a variable wavelength, and the optical output adjusting unit adjusts an output of the variable wavelength light source.
In accordance with an aspect of the present invention, the incident light supplying unit includes an optical modulating unit for modulating a light, and the optical output adjusting unit adjusts an amplitude of an output of the optical modulating unit.
In accordance with an aspect of the present invention, the incident light supplying unit includes an optical attenuating unit for attenuating the light, and the optical output adjusting unit adjusts an attenuating ratio of the optical attenuating unit.
According to the present invention an optical characteristic measuring apparatus for measuring a characteristic of a light transmitting objective includes: an incident light supplying unit for supplying incident light to the objective a waveform distortion measuring unit for measuring a waveform distortion of the light transmitted from the objective; a multi-wavelength light adding unit for adding multi-wavelength light of a plurality of wavelengths to the incident light; and a multi-wavelength light adjusting unit for adjusting an output of the multi-wavelength light so that the waveform distortion measured by the waveform distortion measuring unit falls within a predetermined range.
Further, in an optical characteristic measuring apparatus so constructed, when incident light of a wavelength in a narrow range is supplied to the objective, a distortion in a waveform of the transmitted light is caused. However, by adjusting the output of the multi-wavelength light of the plurality of wavelengths, being added to the incident light, the incident light of a wavelength in a wide range can be supplied to the objective. Therefore, it is possible to reduce the waveform distortion of the outgoing light.
In accordance with an aspect of the present invention, the multi-wavelength light is noise light.
In accordance with an aspect of the present invention, the multi-wavelength light adding unit is an operational amplifier, an input of which is not given, and the multi-wavelength light adjusting unit is a light attenuating unit for attenuating the output of the operational amplifier and changing an attenuating ratio in accordance with the waveform distortion.
In accordance with an aspect of the present invention, the waveform distortion is kept at minimum.
In accordance with an aspect of the present invention, the incident light supplying unit includes: a variable wavelength light source for generating light of a variable wavelength; a power supply of a modulating frequency use to modulate the variable wavelength light; and an optical modulating unit for modulating the variable wavelength light into the modulating frequency. The apparatus further includes: a photoelectric converting unit for photoelectrically converting the transmitted light a phase comparing unit for measuring a phase difference between an output of the photoelectric converting unit and an output of the power supply and a characteristic calculating unit for obtaining a group delay or a wavelength dispersion of the objective from the phase difference.
According to the present invention an optical characteristic measuring method for measuring a characteristic of a light transmitting objective includes: supplying an incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; and adjusting an output of the incident light so that the waveform distortion measured in the waveform distortion measuring step falls within a predetermined range.
According to the present invention an optical characteristic measuring method for measuring a characteristic of a light transmitting objective includes: supplying incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; adding multi-wavelength light having combined therein a plurality of wavelengths to the incident light; and adjusting an output of the multi-wavelength light so that the waveform distortion measured in the waveform distortion measuring step falls within a predetermined range.
According to the present invention a computer-readable medium having a program of instructions for execution by a computer to perform an optical characteristic measuring process for measuring a characteristic of a light transmitting objective is provided. The optical characteristic measuring process includes: supplying incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; and adjusting an output of the incident light so that the waveform distortion measured in the waveform distortion measuring processing falls within a predetermined range.
According to the present invention, a computer-readable medium having a program of instructions for execution by a computer to perform an optical characteristic measuring process far measuring a characteristic of a light transmitting objective is provided. The optical characteristic measuring process includes: supplying incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; adding multi-wavelength light having combined therein a plurality of wavelengths to the incident light; and adjusting an output of the multi-wavelength light so that the waveform distortion measured in the waveform distortion measuring processing falls within a predetermined range.
Here, FIG. 2 (d) shows a relationship between the optical output and the waveform distortion. When the output power of the incident light 3 is set on P0, the waveform distortion becomes a minimum value Smin. The waveform distortion becomes large when the output power of the incident light 3 is either over or under the P0. Accordingly, if the optical output adjusting unit 44 adjusts the ouput of the incident light generated from the optical source system 10 and sets to P0, the waveform distortion can be reduced.